During the past years, with the increase in price of fossil fuels, the interest in recovering energy from high-temperature or high-pressure gases has increased. However, the available devices are not as efficient as can be and suffer from certain limitations that are discussed later.
As any high-temperature or high-pressure gas is a potential resource for energy recovery, generator-loaded expanders or turbines or turboexpanders can be custom engineered to recover a large amount of useful energy available in the process.
One field in which turboexpanders play a role is waste heat recovery. Waste heat can be converted to useful energy with a turboexpander-generator alone or as a component in a more complex system. Potential heat sources include: tail gas from industrial furnaces or combustion engines, waste vapor from industrial furnaces or combustion engines, waste vapor from chemical and petrochemical processes, and solar heat from flat or parabolic reflectors. Exhaust gases are hot and may contain solvents or catalysts. An expander can not only recover energy and cool down exhaust gases which vent to the atmosphere, it can also separate solvents or catalysts.
Another field in which turboexpanders are useful is the extraction of useful work in pressure letdown applications. In pressure letdown applications, such as the merging of two transmission pipelines at different pressures or at a city gate of a gas distribution system, a turboexpander-generator can reduce the pressure of large volume gas streams while at the same time recovering energy in the form of electric power. An expander can therefore be a profitable replacement for other pressure regulating equipment such as control valves and regulators.
A turboexpander, also referred to as a turbo-expander or an expansion turbine, is a centrifugal or axial flow turbine through which a high-pressure gas is expanded to produce work that is often used to drive a compressor. Because work is extracted from the expanding high-pressure gas, the gas expansion may approach an isentropic process (i.e., a constant entropy process) and the low pressure exhaust gas from the turbine is at a low temperature, sometimes as low as −90° C. or less.
Because of the low temperature generated, turboexpanders are widely used as sources of refrigeration in industrial processes such as the extraction of ethane and the formation of liquefied natural gas (NGLs) from natural gas, the liquefaction of gases (such as oxygen, nitrogen, helium, argon and krypton) and other low-temperature processes.
Such an example of a turboexpander is shown in FIGS. 1 and 2, which are reproduced from U.S. Pat. No. 5,851,104, the entire content of which is incorporated herein by reference. FIG. 1 shows a variable nozzle arrangement in a radial inflow turbine. The radial inflow turbine has a housing 10 with an annular inlet 12. A fixed circular plate 16 is positioned to one side of the annular inlet 12. The nozzle adjustment system is provided to the other side of the annular inlet 12. An adjusting ring 32 is arranged radially outwardly of a clamping ring 22. The adjusting ring 32 is able to rotate about the clamping ring 22 which is prevented from rotating by nozzle pivot pins 30 anchored in the fixed circular plate 16.
Vanes 40 are located about the annular inlet 12. These vanes are positioned between the fixed circular plate 16 on one side and the clamping ring 22 and adjusting ring 32 on the other. The vanes 40 are configured to provide a streamlined flow path therebetween. This path may be increased or decreased in cross-sectional area based on the rotational position of the vanes 40. The vanes 40 are pivotally mounted about the nozzle pivot pins 30. The relative positioning of the vanes 40 with respect to the clamping ring 22 is illustrated by the superimposed phantom line in FIG. 2.
In the 104′ patent, the nozzle adjusting mechanism includes a cam and cam follower mechanism. Cam followers 44 are displaced laterally from the axis of the pins 30 and are fixed by shafts in the vanes 40, respectively, as shown in FIG. 2. The cam followers 44 rotate about the shafts freely. To cooperate with the cam followers 44, cams in the form of biased slots 48 are arranged in the adjusting ring 32. They are sized to receive the cam followers 44 so as to allow for free-rolling movement as the adjusting ring 32 is rotated.
The above described arrangement of the vanes 40, cam followers 44, biased slots 48 and the adjusting ring 32 make the opening of the vanes 40 linearly dependant on a rotation of the adjusting ring 32. In other words, a given rotation of the adjusting ring 32 produces the same preset rotation of the vanes 40 irrespective of whether the vanes 40 are near an opened position, are in an opened position, are near a closed position or are in a closed position. This constant rotation of the vanes 40 with the rotation of the adjusting ring 32 does not allow for any varied sensitivity in the adjustment of the position of vanes 40.
In some traditional turboexpanders an adjusting ring directly slides on vanes, which produces friction and may damage part of the adjusting ring and/or vanes. The same sliding motion may prematurely wear the adjusting ring and/or vanes. Also, in some traditional turboexpanders two forces are applied at different locations of the adjusting ring that create an undesired torque. A first force acts on the adjusting ring due to a mechanism that actuates the adjusting ring while a resistance force occurs on the adjusting ring at a connection between the adjusting ring and vanes. The occurrence of these two forces on the adjusting ring create a torque, which tends to press the adjusting ring on part of the vanes, introducing further friction and wear to the components of the turboexpander.
Accordingly, it would be desirable to provide devices and methods that avoid the afore-described problems and drawbacks.